Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

Hess Law Lab

PDF – Paper 31 2011

 

Enthalpy formation of Magnesium oxide using Hess Law

You will determine, using Hess’ Law, the enthalpy change, ΔH1, for the reaction of magnesium
with oxygen to form magnesium oxide.

Mg(s) + ½O2(g) MgO(s)

Materials: 

You List them

Chemicals:

  • Sulfuric acid 1 M – 200 cm3
  • Magnesium Turnings (2 containers of around 1 g each) 
  • Magnesium Oxide (2 containers of around 1.25 g each) 

Part 1 – Reaction of Metallic Magnesium with Sulfuric Acid

Calculate the enthalpy change ΔH2, in kJ/mol for the following reaction: 

Mg(s) + H2SO4(aq) MgSO4(aq) + H2(g)

  • Place the styrofoam cup into the 250 cm3 beaker.
  • Using a measuring cylinder, transfer 25 cm3 of acid into the plastic cup.
  • Tilt the beaker so that the bulb of the thermometer is covered by the solution.
  • Measure and record the initial temperature of the solution.
  • Carefully add the Mg turnings in one of the containers into the plastic cup.
  • Stir the mixture constantly with the thermometer.
  • Measure and record the highest temperature obtained.
  • Empty and rinse the plastic cup and dry it with a paper towel.
  • Repeat the experiment using the second portion of Magnesium turnings
  • Record all your readings in an appropriate form.
  • Calculate the mean temperature rise.
  • Show your working and express your answers to three significant figures.
  1. Using the mean temperature rise above, calculate the mean heat energy produced
    in the reaction. (Assume that 4.3 J are required to raise the temperature of 1.0 cm3 of any
    solution by 1.0 °C.)
  2. Calculate the enthalpy change, ΔH1, in kJ mol–1, for the following reaction.

Mg(s) + H2SO4(aq) MgSO4(aq) + H2(g)
You should assume that the magnesium in your reaction is in excess.

Part 2 – Reaction of Magnesium Oxide with Sulfuric Acid

Calculate the enthalpy change, ΔH3, in kJ mol–1, for the following reaction.

MgO(s) + H2SO4(aq) MgSO4(aq) + H2O(l)

  • Using a measuring cylinder, transfer 50 cm3 of FA 4 into a 250 cm3 beaker.
  • Place the beaker on a tripod and gauze, or a hot plate, and heat gently until the temperature of the acid reaches  between 45 °C–60 °C.
    • Support a styrofoam (plastic) cup in a 250 cm3 beaker.
    • Transfer all the solution of hot FA 4 into the plastic cup.
    • Stir and record the temperature of hot FA 4.
    • Immediately add all the FA 6 to the FA 4 in the plastic cup.
    • Stir the mixture constantly with the thermometer.
    • Record the highest temperature obtained.
    In the space below, record all your readings in an appropriate form.

Calculation
Show your work and express your answers to three significant figures.

  1. Calculate the heat energy produced in the reaction.
    (You may assume that 4.3 J are required to raise the temperature of 1.0cm3 of any solution by 1.0 °C.)
  2. Calculate the enthalpy change, ΔH3, in kJ mol–1, for the following reaction.
    MgO(s) + H2SO4(aq) MgSO4(aq) + H2O(l)
    You should assume that the magnesium oxide in your reaction is in excess.
  3. The enthalpy change for the following reaction is –286 kJ mol

    H2(g) + ½O2(g) H2O(l) ΔH = –286 kJmol–1

Use the Hess’ Law cycle given below to calculate ΔH1, the enthalpy change for the reaction of magnesium with oxygen.

 

 

Suggest one improvement to the method by which heat losses from your apparatus could have been reduced.

Analysis questions:

#1: Calculate the enthalpy for this reaction:

2C(s) + H2(g) —> C2H2(g)         ΔH° = ??? kJ

Given the following thermochemical equations:

C2H2(g) + 5⁄2O2(g) —> 2CO2(g) + H2O(ℓ)        ΔH° = −1299.5 kJ

C(s) + O2(g) —> CO2(g)                                      ΔH° = −393.5 kJ

H2(g) + 1⁄2O2(g) —> H2O(ℓ)                               ΔH° = −285.8 kJ

ANSWER: +226.7. kJ 

#2: Calculate the enthalpy of the following chemical reaction:

CS2(ℓ) + 3O2(g) —> CO2(g) + 2SO2(g)

Given the following thermochemical equations:

C(s) + O2(g) —> CO2(g)                ΔH = −393.5 kJ/mol

S(s) + O2(g) —> SO2(g)                ΔH = −296.8 kJ/mol

C(s) + 2S(s) —> CS2(ℓ)                 ΔH = +87.9 kJ/mol

ANSWER: 1075 kJ

#3: Given the following data:

SrO(s) + CO2(g) —> SrCO3(s)      ΔH = −234 kJ

2SrO(s) —> 2Sr(s) + O2(g)          ΔH = +1184 kJ

2SrCO3(s) —> 2Sr(s) + 2C(s, gr) + 3O2(g)ΔH = +2440 kJ

Find the ΔH of the following reaction:

C(s, gr) + O2(g) —> CO2(g)

ANSWERl -394 kJ

#4: Using the following thermochemical equations, calculate the standard enthalpy of combustion for one mole of liquid acetone (C3H6O).

3C(s) + 3H2(g) + 1⁄2O2(g) —> C3H6O(ℓ)    ΔH° = −285.0 kJ

C(s) + O2(g) —> CO2(g)    ΔH° = −394.0 kJ

H2(g) + 1⁄2O2(g) —> H2O(ℓ)  ΔH° = −286.0 kJ

ANSWER: -1755 kJ

#5  The standard enthalpy change of formation of propane is impossible to measure directly. That is because carbon and hydrogen will not directly react to make propane. However, standard enthalpy changes of combustion are relatively easy to measure.

C3H8(g)            ΔH1 = −2219.9 kJ

C(s, gr)              ΔH2 = −393.5 kJ

H2(g)                 ΔH3 = −285.8 kJ

Answer: -103.8 kJ